A recent advancement in semiconductor technology involves the replacement of dielectric materials used for insulating interconnects with low-k dielectric materials. Low-k dielectric materials are currently being integrated as interlevel dielectric materials. The three main categories of low-k dielectric materials include: inorganic (SiO2 based material); hybrid (organic functionalized inorganic matrix), and organic materials. This shift to using low-k dielectric materials has required photoresist stripping to evolve to meet higher requirements for cleanliness and residue removal, without adding cost and affecting throughput.
By using the low-k dielectric materials for insulating the interconnects, smaller geometry interconnect structures can be built resulting in faster integrated circuits. Porous low-k dielectric materials are a particular class of these low-k dielectric materials. When etching lines and vias in the porous low-k dielectric materials, silanol groups tend to form on surfaces within the lines and the vias. The silanol groups also tend to form in the voids of the porous low-k dielectric materials adjacent to the lines and the vias.
In the case of low-k dielectric inorganic and hybrid materials, cleaning of these materials presents a challenge in that traditional cleaning formulations are designed to remove etch residues through dissolution of the residue or slight etching of the dielectric to release the residue. But, with low-k dielectric materials, the increased surface area due to their porosity greatly increases their sensitivity to these cleaning formulations, reducing the selectivity of the formulation to the etch residue. Also, traditional dry cleaning methods such as ashing have unacceptable shortcomings because the ashing plasma tends to affect the organic content of the hybrid materials, thereby increasing the dielectric constant.
Currently, there are two basic systems in use: wet and dry. Dry is typically used for stripping and wet is usually used for cleaning. Wet systems use acids, bases or solvents, requiring several processing steps for residue removal. Dry systems are the preferred choice when dealing with organic photoresist material. Even when dry stripping systems are utilized, post-strip wet processing is still required to remove inorganic residues that the dry systems leave behind.
In semiconductor fabrication, a low-k dielectric material layer is generally patterned using a photoresist mask in one or more etching and ashing steps. These films, after etching or due to their physical nature, tend to have large numbers of silanol functionalities on their surfaces, and, due to their porous nature, present a large surface area of material to a cleaning formulation during cleaning. This presents the problem of substantial etching of the low-k dielectric material film with many cleaning formulations, often to the point of destroying the low-k dielectric material film.
To remove these silanol groups, the etch and photoresist residue in the lines and the vias, and the bulk photoresist from an exposed surface of the low-k dielectric material, a cleaning process is performed following the etching of the lines and the vias. In this cleaning process, a weak etchant is typically employed to remove a monolayer of the low-k dielectric material in order to release the etch residue, the photoresist, and the bulk photoresist. It has been found that this cleaning process results in an unacceptably high etch rate of the porous low-k dielectric materials. This is even true when the porous low-k dielectric materials are exposed to a weak etchant. Where the silanol groups exist, it has been found that significantly more than the monolayer of the low-k dielectric material is removed by the weak etchant.
Current high-dose implant cleaning has problems. When utilized, the resist gets heavily implanted, the hydrogen is driven from the resist's top third, and an extremely carbonized layer is produced. This carbonized layer is hard to remove and does not etch as quickly. Further, bulk resist with volatile components still exist underneath.
Even if normal stripping is utilized, there is a pressure build-up resulting in popping and blistering while cleaning at a slower rate. This not only contaminates the chamber, but these carbonized chunks also bond with exposed areas of the wafer's surface. In addition, standard high temperature oxygen-based plasmas do not work for low-k dielectric material cleaning. These high temperature and high-oxygen environments oxidize and degrade film integrity and low-k dielectric material properties.
What is needed is a method of treating porous low-k dielectric materials subsequent to etching and prior to cleaning which reduces the presence of silanol groups in the porous low-k dielectric materials. The challenge is to ensure the cleaning method is aggressive enough to clean the surface efficiently, without etching or altering the low-k material.